This invention relates to a jet engine and, more particularly, to a hypersonic ducted engine having at least one combustion chamber and a propelling nozzle connected behind it, with a nozzle outer wall which is fixed to the engine and is divergent in the direction of the nozzle outlet end. An axially movable propelling nozzle adjusting part for the bounding of a hot gas flow duct is provided with a variable cross-section,
From U.S. Pat. No. 4,527,388, a ducted fan engine is known which comprises a convergent propelling nozzle and a mushroom-shaped central body. The central can be axially adjusted for changing the nozzle throat cross-section in the interior of the nozzle. The outer cold-air flow guided around the basic engine can be admixed to the hot-gas flow carried through the propelling nozzle by way of an air outlet duct which is arranged at the nozzle outlet end and has an outer boundary wall which convergently encloses the propelling nozzle. The boundary wall can be axially adjusted for the regulating of the effective air duct cross-section and thus of the cold-air/hot-gas flow ratio independently of the central body. A ducted fan engine of this type comprising a propelling nozzle which is convergent at the outlet end and can be adjusted for an adaptation to different double-flow quantity ratios for an operative range which is limited with respect to the flying altitude and flying speed is not the object of the invention.
Furthermore, from German Patent Documents DE 40 10 471 A1 and DE 40 12 212 A1, hypersonic ducted engines of the claimed type are known which, at low flying speeds of up to approximately Mach 3, operate as a turbojet engine and in the upper speed range of between Mach 3 and approximately Mach 7 operate as a ramjet engine. Therefore, these engines require propelling nozzles which, with respect to the narrowest area of cross-section as well as with respect to the expansion ratio, have a very high variation range and, for this purpose, in addition to a mushroom-shaped central body which can be adjusted for changing the nozzle throat cross-section axially between the inlet and outlet cross-section of the divergent outer wall of the nozzle, have swivellable rear flaps at the nozzle outlet end. In the ramjet operation, the upper flap is placed in the fully swivelled-out position onto an expansion ramp on the aircraft assigned to the engine. In the turbojet operation, when the rear flaps are swivelled in to a position narrowing the hot-gas flow duct, controls open an air duct via which the boundary layer air is blown from the engine inlet into the hot-gas jet in the area of the expansion ramp downstream of the upper rear flap. The problems in the case of engines of this type are, on the one hand, the high mechanical constructional and mainly sealing expenditures for the swivellable propelling nozzle flaps and, on the other hand, the reduction of propulsion which is particularly pronounced in critical flying conditions. Thus, in the transonic operation, there is an extremely disturbing vacuum zone on the underside of the engine and an increase of the tail resistance which are caused by a wake forming during the swivelled-in condition of the nozzle flaps on the divergent end of the propelling nozzle outer wall because of a separation of the ambient flow. In addition, in the starting and/or change-over phase from the turbo-operation to the ramjet operation, there are clear losses of propulsion because the achievable cross-sectional changes of the hot-gas flow duct are not sufficient for a propulsion-optimal adaptation to the drastically changing hot-gas volume flow.
There is therefore needed a jet engine of the initially mentioned type developed in such a manner that the geometry of the propelling-nozzle-limited hot gas flow duct can be adapted in a constructively simple manner with a high propulsion efficiency to extensively differing operating conditions, as they occur particularly in the case of the described hypersonic ducted engines.
According to the present invention, these needs are met by an air-breathing jet engine, particularly a hypersonic ducted engine, having at least one combustion chamber and a propelling nozzle connected behind it, with a nozzle outer wall which is fixed to the engine and is divergent in the direction of the nozzle outlet end. An axially movable propelling nozzle adjusting part for the bounding of a hot gas flow duct is provided with a variable cross-section. On the outlet end of the nozzle outer wall fixed to the engine, a secondary-air injection is provided. The secondary-air injection can be connected selectively as a function of the axial position of the propelling nozzle adjusting part, by way of an air outlet duct. The air outlet duct is formed in the moved-in condition of the adjusting part and encloses the nozzle outer wall in a ring-shaped manner. The adjusting part is constructed as an annular wall which, in the moved-in condition, bounds the air outlet duct on the outer circumferential side and, in the moved-out position, is constructed as a propelling nozzle lengthening which connects essentially in the corresponding shape divergently to the outlet end of the nozzle outer wall.
According to the present invention, using the axially movable, bell-shaped divergent propelling nozzle lengthening, in conjunction with the claimed central-body adjustability, in a constructionally simple, low-leakage manner, a course of the cross-section of the hot-gas flow duct limited by the nozzle contour that is highly variable with respect to the outlet surface size as well as with respect to the nozzle throat surface size, is obtained on the one hand. Thus, the propelling nozzle can be adapted without any problems with high propelling efficiency to extremely different nozzle pressure conditions. For example, in the case of the above-mentioned change-over from the turbo-operation to the ramjet operation. On the other hand, in conjunction with the annular secondary-air injection connected in the position of the propelling nozzle lengthening withdrawn from the hot-gas jet boundary, at the outlet end of the fixed outer wall of the nozzle, the wake effect which is particularly pronounced there in the transonic range, is effectively reduced. As a result, the tail resistance and the propulsion-reducing vacuum zone are clearly reduced on the underside of the engine, with the additional constructional simplification that the axially movable nozzle lengthening part takes over the expansion of the outer wall of the nozzle not only in the moved-out position but, in addition, in the moved-in condition, also takes over the outer bounding of the air duct and the control of the secondary-air injection.
For the purpose of a further improvement of the propulsion effect, according to an advantageous further development of the invention, the nozzle lengthening part is axially adjustable to a limited degree in the moved-in condition for the control of the air-duct outlet cross-section. This is to achieve, by means of the air injection, an additional gain in thrust that is as large as possible. Also, for reasons of a uniform secondary-air distribution and a constructionally advantageous sealing for the secondary-air supply to the air outlet duct, a distributor ring is preferably provided which reaches around the outer wall of the nozzle. The ring is fixed to the engine, is closed in the circumferential direction and has an annular gap which is open in the direction of the nozzle outlet end. In the moved-in condition of the adjustable nozzle lengthening part, the ring is sealingly enclosed by the latter, in which case the sealing between the distributor ring and the nozzle lengthening part, for the purpose of the preferred adjustability of the air duct cross-section, expediently takes place by means of a sliding seal. The sliding seal acts in the moved-in condition of the lengthening part within a limited axial stroke. The distributor ring is arranged in an advantageous manner with respect to its mounting preferably within the range of the smallest outside diameter of the nozzle wall.
For the disconnecting of the secondary-air injection not required in the moved-out position of the nozzle lengthening part, shut-off elements are expediently provided upstream of the air outlet duct or of the distributor ring.
In the case of engines which operate by a boundary layer suction on the engine inlet in flying conditions without any hot gas duct lengthening via the propelling nozzle part which can be moved out, particularly hypersonic engines in the turbo-operation, in a particularly preferred manner, the boundary layer air is used for the secondary-air injection on the divergent end of the outer wall of the nozzle.
In order to further increase the variation range of the achievable nozzle throat cross-section changes, the central body is adjustably arranged between the narrowest cross-section of the outer nozzle wall fixed to the engine and the outlet plane of the nozzle lengthening in the moved-out position of the divergent nozzle lengthening by means of its largest cross-section.
Finally, a further controlled expansion of the thrust jet is achieved on the downstream side of the moved-out propelling nozzle lengthening, preferably using an expansion ramp fixed to the aircraft which bounds the thrust jet on one side.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.